April 13, 2009 -- People who smoke both cigarettes and marijuana have a greater risk for developing the progressive lung disease COPD than cigarette smokers who don't smoke pot, a new study shows.

Smokers in the study who reported using both tobacco and marijuana were three times as likely as nonsmokers to have clinically confirmed COPD (chronic obstructive pulmonary disease); people who smoked only cigarettes had a slightly lower risk.

The study is among the first to suggest a synergistic relationship between marijuana and tobacco use among older people who are most at risk for COPD.

"This effect suggests that smoking marijuana may act as a primer, or sensitizer, in the airways to amplify the adverse effects of tobacco on respiratory health," says study researcher Wan C. Tan, MD, of the University of British Columbia and St. Paul's Hospital in Vancouver, Canada.

COPD Risk

About 12 million Americans are currently living with a diagnosis of COPD; an equal number are believed to have the disease and not know it, according to the National Heart, Lung and Blood Institute.

Largely caused by cigarette smoking, COPD is the fourth leading cause of death in the U.S.

While the link between tobacco and COPD is well established, far less is known about the impact of marijuana use on the lungs.

Some studies have found that even short-term heavy marijuana smoking can worsen lung function, while others have not shown this association.

Even less is known about the combined effects of smoking cigarettes and pot, Tan tells WebMD.

The study by Tan and colleagues included 878 residents of Vancouver, Canada participating in a larger investigation examining COPD prevalence.

Participants were considered tobacco smokers if they had smoked at least 365 cigarettes in their lifetime, and were considered marijuana smokers if they reported having ever smoked pot. The researchers defined "substantial" marijuana use as having smoked at least 50 marijuana cigarettes.

The average cigarette smoker in the study had smoked for 16 years, while the self-described pot smokers had smoked an average of 84 marijuana cigarettes.

When COPD was clinically confirmed though a diagnostic method known as spirometric testing:

The incidence of COPD among participants who smoked cigarettes alone was 2.7 times higher than among nonsmokers.

The incidence of COPD was 2.9 times higher among participants with a history of smoking both cigarettes and pot, even after controlling for other risk factors for the pulmonary disease.

COPD risk among people who smoked marijuana, but not tobacco, was slightly higher than among nonsmokers, but the increase was not statistically significant.

The study appears in the April 14 issue of the Canadian Medical Association Journal.

Marijuana and COPD

Tan tells WebMD that the fact that no link was found between pot smoking alone and COPD does not mean that marijuana is not harmful to the lungs.

She says the study was probably too small, with too few participants who identified themselves as marijuana smokers alone, to show an association.

"We can't say that smoking marijuana alone does not increase the risk for COPD," she says. "We definitely need bigger studies to explore the question."

Donald P. Tashkin, MD, a researcher on marijuana and pulmonary disease, agrees that firm conclusions cannot be made about the association between marijuana smoking and COPD based on the research that has been done.

He notes that the studies have been limited by their small size and uncertain accuracy of self-reported marijuana use.

But in an editorial accompanying the study, Tashkin writes that the research makes an increasingly compelling case that smoking marijuana alone is not a major risk factor for lung disease.

Tashkin's own research, reported in 2006, found no link between marijuana use and lung cancer.

"Given the consistently reported absence of an association between use of marijuana and abnormal [lung function] or signs of macroscopic emphysema, we can be close to concluding that smoking marijuana by itself does not lead to COPD," he writes.